IB Chemistry SL

Revision Notes

4.1.11 Giant Covalent Structures

Giant covalent structures

Covalent lattices

  • Covalent bonds are bonds between nonmetals in which electrons are shared between the atoms
  • In some cases, it is not possible to satisfy the bonding capacity of a substance in the form of a molecule; the bonds between atoms continue indefinitely, and a large lattice is formed. There are no individual molecules and covalent bonding exists between all adjacent atoms
  • Such substances are called giant covalent substances, and the most important examples are C and SiO2
  • Graphite, diamond, buckminsterfullerene and graphene are allotropes of carbon

Diamond

  • Diamond is a giant lattice of carbon atoms
  • Each carbon is covalently bonded to four others in a tetrahedral arrangement with a bond angle of 109.5o
  • The result is a giant lattice with strong bonds in all directions
  • Diamond is the hardest substance known
    • For this reason it is used in drills and glass-cutting tools

The structure of diamond

Graphite

  • In graphite, each carbon atom is bonded to three others in a layered structure
  • The layers are made of hexagons with a bond angle of 120o
  • The spare electron is delocalised and occupies the space in between the layers
  • All atoms in the same layer are held together by strong covalent bonds, and the different layers are held together by weak intermolecular forces

The structure of graphite

Buckminsterfullerene

  • Buckminsterfullerene is one type of fullerene, named after Buckminster Fuller, the American architect who designed domes like the Epcot Centre in Florida
  • It contains 60 carbon atoms, each of which is bonded to three others by single covalent bonds
  • The fourth electron is delocalised so the electrons can migrate throughout the structure making the buckyball a semi-conductor
  • It has exactly the same shape as a soccer ball, hence the nickname the football molecule

The structure of buckminsterfullerene

Graphene

  • Some substances contain an infinite lattice of covalently bonded atoms in two dimensions only to form layers. Graphene is an example
  • Graphene is made of a single layer of carbon atoms that are bonded together in a repeating pattern of hexagons
  • Graphene is one million times thinner than paper; so thin that it is actually considered two dimensional

The structure of graphene

Silicon(IV)oxide

  • Silicon(IV)oxide is also known as silicon dioxide, but you will be more familiar with it as the white stuff on beaches!
  • Silicon(IV)oxide adopts the same structure as diamond –  a giant structure made of tetrahedral units all bonded by strong covalent bonds
  • Each silicon is shared by four oxygens and each oxygen is shared by two silicons
  • This gives an empirical formula of SiO2

The structure of silicon dioxide

Properties of giant structures

  • Different types of structure and bonding have different effects on the physical properties of substances such as their melting and boiling points, electrical conductivity and solubility

Covalent bonding & giant covalent lattice structures

  • Giant covalent lattices have very high melting and boiling points
    • These compounds have a large number of covalent bonds linking the whole structure
    • A lot of energy is required to break the lattice
  • The compounds can be hard or soft
    • Graphite is soft as the forces between the carbon layers are weak
    • Diamond and silicon(IV) oxide are hard as it is difficult to break their 3D network of strong covalent bonds
    • Graphene is strong, flexible and transparent which it makes it potentially a very useful material
  • Most compounds are insoluble with water
  • Most compounds do not conduct electricity however some do
    • Graphite has delocalised electrons between the carbon layers which can move along the layers when a voltage is applied
    • Graphene is an excellent conductors of electricity due to the delocalised electrons
    • Buckminsterfullerene is a semi-conductor
    • Diamond and silicon(IV) oxide do not conduct electricity as all four outer electrons on every carbon atom is involved in a covalent bond so there are no free electrons available

Characteristics of giant covalent structures table

Exam Tip

Although buckminsterfullerene is included in this section it is not classified as a giant structure as it has a fixed formula, C60

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